Land Use Effects on Carbon Storage in Thailand Tropical Ecosystems

Measurements of stable isotopes of C have proved to be of value in estimating soil organic C turnover times and in partitioning soil organic carbon (SOC) from different sources. Typically, the contrast between sources and estimates of C turnover have been studied in ecosystems where C-3 photosynthetic plants such as hardwoods have been replaced by C-4 photosynthetic plants from agriculture such as corn or sugarcane. Here we report concentrations and stable C isotope ratios of SOC from Thailand coastal mangrove forests and intrusive coastal aquaculture in the form of shrimp and wastewater treatment ponds. There are clear changes in both magnitude and ¹³C/¹²C of SOC at former mangrove sites which have been altered to make ponds for shrimp farming and wastewater treatment. For instance, total per cent C from 0-40 cm soil depth (average of four 10 cm layers at 2 sites) was 6.2±2.8% for mature mangrove, while it was only 0.5±0.4% for a 10-year old shrimp pond and 1.3±0.4% for an 8-year old water treatment pond. Previous studies of mangrove organic C balance have indicated that these inter-tidal forest ecosystems are a sink for C and that significant C is vested in both above- and below-ground biomass and stored in sediments. Mangrove forest disturbance by human activities clearly has the potential to affect C storage. Our data indicates that stable C isotope tracing will be of value in tracking changes in coastal forest-aquaecosystems just as it has been for forest-agroecosystems